The proposed work deals with the etiology, pathology, pathogenesis and long term sequelae of Host Versus Graft syndrome, a new allogenic disease occurring in parent strain mice neonatally inoculated with semi-allogenic F1 hybrid spleen cells. Morphological studies will be correlated with quantitative analyses immunoglobulins and in vitro tests of cellular and humoral immunity. An isoenzyme marker will be used to quantitate persistence of donor cells at various stages of the disease.